Photopolymerizable monomers for use in the photopolymerizable inkjet ink include (meth)acrylic acid ester compounds, acrylamide compounds, oxetane compounds, epoxy compounds, etc., and the (meth)acrylic acid ester compounds and the acrylamide compounds are preferably used because of being widely available in significant quantities, and inexpensively and easily obtainable.
However, many of the (meth)acrylic acid ester compounds and the acrylamide compounds are poisonous. Some of them have low oral toxicity and skin irritancy, but those having low viscosity used for the inkjet ink are not safe materials in skin sensitization, occasionally causing contact allergy.
Meanwhile, methods of adsorbing a polymeric dispersant for well dispersing a pigment such as carbon black in an inkjet ink are known. In addition, many references such as Japanese published unexamined applications Nos. 10-7968, 10-237367 and 10-251589 disclose specifying a molecular weight of the polymeric dispersant for better dispersibility are known, but lowering viscosity is not mentioned at all therein. As Einstein methods show, the viscosity of a dispersion depends on a volume fraction of a pigment, and the thickness of an adsorption layer needs to be as thin as possible to prevent the volume of the pigment from increasing due to adsorption of the polymeric dispersant. Even though the dispersant has a low molecular weight, a thick adsorption layer is occasionally formed unless adsorption of the polymeric dispersant is controlled, failing in obtaining low viscosity.
Japanese published unexamined application No. 10-237368 discloses a method of decreasing increase of viscosity when blending a pigment. However, this needs a process of centrifugally removing an extra polymeric dispersant, resulting in a complicated process of preparing an ink.
Because of these reasons, a need exists for a low-viscosity photopolymerizable inkjet black ink using a photopolymerizable monomer safe in skin sensitization.